The present invention generally relates to an image processing apparatus, and more particularly to an image processing apparatus in which an undesired moire is prevented from occurring in an output image reproduced from a screened halftone image such as a photograph.
In digital copying machines or facsimile machines, a multilevel image signal is generated from an original image by a scanner having a CCD line sensor. The multilevel image signal is processed by an image processor so as to produce a set of picture elements describing a gray level of an output image to be copied, transmitted or received. When a multilevel image signal is generated from a dot area of an original image, it is known that the moire is likely to occur in the output image due to interference between the sampling frequency of the CCD line sensor and the dot frequency of the dot area, thus degrading the image quality. The original image is a composite image in which a dot image and a line image coexist. The dot image refers to a photograph or the like which is described by dots, while the line image refers to a character or the like which is described by lines.
As an example, FIG. 1 shows a case where a multilevel image signal is generated from a screened halftone area having a dot frequency of 133 l/i (lines per inch) by a CCD line sensor having a sampling frequency of 400 dpi (dots per inch). As shown in FIG. 1, due to the interference between the sampling frequency and the dot frequency, the amplitude of the multilevel image signal periodically changes in spite of a uniform dot density of the original image.
FIG. 2 shows multilevel image data after a multilevel image signal generated from a screened halftone image of uniform dot density is processed through an MTF (modulation transfer function) correction. The multilevel image data represents a gray level of each pixel of a set of pixels aligned in the main scanning direction and in the sub scanning direction, and such a gray level is respectively indicated by a numeral within each partition (showing a pixel) in FIG. 2. It is difficult to appreciate the presence of the moire when the multilevel image data of FIG. 2 is partially viewed. However, if the changes in the gray level corresponding to pixels aligned along a scanning line of the scanner are viewed, it is apparent that the amplitude of a multilevel image signal periodically changes. FIG. 3 shows changes in gray level of a multilevel image signal Din, generated by the scanner having the CCD line sensor, with respect to pixels aligned in the main scanning direction. In FIG. 3, it is apparent that the moire occurs in the output image.
In an image processing apparatus, a smoothing process is performed for filtering of an image signal, and the MTF correction process is performed for sharpening an edge portion of an image or intensifying an edge portion thereof. It is known that the occurrence of the moire is reduced to a certain extent if the smoothing process is performed with respect to a local region of an image. However, there is a problem in that, if the smoothing process is performed for a composite image where a dot area such as a dot photograph and a line area such as a character coexist, the line area conversely becomes dim or blurs in the output image due to the smoothing process, thus degrading the picture quality. Moreover, it is known that the moire is also intensified in the output image if the MTF correction process is performed. Thus, it is desirable to prevent the moire from occurring in the output image even when the MTF correction process is performed.